The invention relates to a lithography device for carrying out projection lithography by means of charged particles, which device includes an imaging particle-optical system for imaging a lithographic object structure on a lithographic imaging surface.
A device of this kind is known from an article in Proceedings SPIE xe2x80x9cElectron-Beam Sources and Charged-Particle Opticsxe2x80x9d, Jul. 10-14, 1995, by W. K. Waskiewicz et al., entitled xe2x80x9cElectron-Optical Design for the SCALPEL Proof-of-Concept Toolxe2x80x9d, published in SPIE, Vol. 2522, 1995.
Particle-optical imaging, notably electron optical imaging, can be used for the lithographic manufacture of very small structures, such as integrated electronic circuits or masks for such circuits, with a resolution which is less than the wavelength of light.
The imaging of a lithographic object structure on a lithographic imaging surface by means of electrons can in principle be carried out in two ways: sequentially and non-sequentially. In the case of sequential imaging, the emissive surface of an electron source, or a part thereof, is imaged, at a strongly reduced scale, on the lithographic imaging surface on which the lithographic structure to be formed is to be provided. This image of the electron source (the xe2x80x9cspotxe2x80x9d) is displaced across the object by means of, for example deflection coils, the electron beam being blanked or not during said displacement. The pixels of the pattern to be imaged are thus sequentially written onto the lithographic imaging surface. As the dimensions of the lithographic structure are larger, significantly more time will be required for the scanning writing of this structure, i.e. an increase in time in proportion to the surface area of the structure. Because nowadays in the integrated circuit technique there is a strong tendency to image increasingly larger structures, the throughput during the production of integrated circuits decreases strongly, so that this method of imaging is becoming increasingly more objectionable.
In the case of non-sequential imaging, the lithographic object structure to be imaged is uniformly irradiated by means of the electron beam and a focusing lens system is used to form an image, reduced or not, of the lithographic object structure on the lithographic imaging surface. The pixels of the pattern to be imaged are thus simultaneously, i.e. not sequentially, projected onto the lithographic imaging surface. Therefore, this method of lithography is also called projection lithography.
The cited article describes a projection lithography method in which a lithographic object structure is imaged on a lithographic imaging surface by means of a system of rotationally symmetrical electron lenses. Such a lithographic object structure can be formed by a (comparatively large) rendition of a lithographic mask which is to be imaged on a lithographic imaging surface in order to derive the actual (much smaller) lithographic mask therefrom. The lithographic object structure to be imaged may also be formed by the actual mask, which is then imaged on the lithographic imaging surface (in that case being a wafer) in order to form integrated circuits therefrom. This known lithographic method is called SCALPELxe2x96xa1 (xe2x80x9cScattering with Angular Limitation Projection Electron-beam Lithographyxe2x80x9d). The imaging system of electron lenses therein is formed by two electron lenses, having a rotationally symmetrical lens field, which together constitute a telescopic system.
In the context of the present invention a telescopic system is to be understood to mean a system of lenses which converts an incident parallel beam into a parallel outgoing beam. The simplest form of such a system consists of two lenses having a common optical axis, the rear focus of one lens being coincident with the front focus of the other lens, as is the case in the cited state-of-the-art system. Projection lithography requires a telescopic system, because a comparatively large lithographic object structure (having a diameter of the order of magnitude of 1 mm) must be completely imaged on the lithographic imaging surface. The edges of said structure should in principle be just as sharp as its center, which means that the imaging defects at the edges of the structure to be imaged may hardly be greater than those at the central parts. This condition can be optimally satisfied only if the imaging system is a telescopic system, so that for the present invention it is of essential importance to perform the imaging by means of such a system.
During the production of integrated circuits by means of projection lithography the throughput is determined by the magnitude of the current in the electron beam whereby the lithographic object structure to be imaged (so the mask to be imaged in the case of IC manufacture) is irradiated. A limit is imposed as regards the current in the electron beam because the electrons in the beam repel one another (the so-called Coulomb interaction), causing an energy spread of the electrons in the beam and distortion of the beam. Both effects are greater as the current in the electron beam is larger, and cause imaging defects by the imaging system. The imaging defects may not exceed a specified value, so that an upper limit is also imposed as regards the current in the beam, and hence also as regards the throughput of the integrated circuits to be produced.
The described repulsion effect is strongest in the part of the electron beam where the spacing of the electrons in the beam is small, i.e. at the area of a cross-over in the electron beam. In the cited state-of-the-art system such a cross-over occurs between said two round lenses which together constitute the telescopic system, that is to say at the area of the coincident focal points of the two lenses. Even though it may occur that cross-overs are also formed in the electron beam ahead of the telescopic system, such cross-overs do not have an effect on the (geometrical) imaging defects, because they are situated in the irradiating part of the beam and do not occur in the imaging beam path between the object (the lithographic object structure) and the image (the lithographic imaging surface).
It is an object of the invention to make said limitation of the current in the electron beam less severe, and hence increase the throughput during the production of integrated circuits.
To this end, the lithography device according to the invention is characterized in that the particle-optical system includes at least five quadrupoles, neighboring quadrupoles of said quadrupoles each time extending perpendicularly to one another, the strength and the location of said quadrupoles being such that the imaging of the lithographic object structure on the lithographic imaging surface is stigmatic, and that the system is telescopic in the x-z plane as well as in the y-z plane.
As is known from particle optics, a quadrupole field exerts a pure converging effect on a beam of charged particles in a first plane containing the optical axis (the converging plane) whereas it has a purely diverging effect in a plane which extends perpendicularly thereto and contains the optical axis (the diverging plane). In the context of the present invention mutually perpendicularly extending quadrupoles are to be understood as a system of quadrupoles in which the converging (diverging) plane of one quadrupole extends perpendicularly to the converging (diverging) plane of another quadrupole.
The invention is based on the recognition of the fact that it is in principle possible to form a stigmatic image by means of more than one quadrupole. Realizing this image by means of a system which is telescopic in the x-z plane as well as in the y-z plane enables the desired high resolution to be achieved at the edges of the image.
Furthermore, the use of quadrupoles offers the additional advantage that, in comparison with round lenses, only little power is dissipated in the windings of the field-generating pole pieces. This is due to well known particle optical fact that a strong lens effect can be achieved for quadrupoles while utilizing a comparatively small number of ampere-turns, so a low power dissipation. Therefore, these windings cause only a slight temperature increase so that mechanical deformations due to thermal expansion remain limited.
It is to be noted that from Japanese patent application 5-286948, published on Jun. 2, 1995 (filed on Nov. 16, 1993, publication No. 7-142318), it is known per se to counteract the Coulomb interaction in an electron beam traversing a lens system by distorting a cross-over by means of quadrupoles which are arranged at the area of the cross-over in order to form a beam structure having an enlarged beam cross-section. The imaging systems known from the cited document, however, are intended to form a spot shape for a sequential imaging application (so the scanning exposure of the lithographic imaging surface). Such known systems include rotationally symmetrical lenses as well as quadrupoles. According to the method described in the cited document the desired spot shape is obtained by imaging a beam-limiting gap onto a second beam-limiting gap and by imaging this assembly in its turn on a lithographic imaging surface. It will be evident that this is not a matter of projection lithography and, therefore, the cited document does not offer any hints as to which steps are required so as to perform projection lithography by means of exclusively quadrupoles which, moreover, also constitute a telescopic system.
When the above-mentioned steps according to the invention are taken, it is also achieved that the image is rotation free, i.e. that the image on the lithographic imaging surface has not been rotated relative to the lithographic object structure, regardless of the excitation of the quadrupoles. This substantially simplifies the alignment of the equipment to be used.
In a preferred embodiment of the lithography device according to the invention, the x magnification from the lithographic object structure to the lithographic imaging surface equals the y magnification. Generally speaking, when quadrupoles are used for a stigmatic image, the magnification in the x direction deviates from that in the y direction. In such cases additional steps would be required so as to compensate for the distortion thus caused, for example by a prior distortion of the object to be imaged. As a result of said steps, a non-distorted image is obtained so that such additional steps can be dispensed with.
In a further preferred embodiment of the lithography device according to the invention, the various parameters of the imaging particle-optical system have the values stated in Claim 3.